Radio frequency (RF) sensors are used to characterize the electrical and magnetic properties of materials, including the properties of fluids, thin films, molecules, particles, biological cells, tissues and organs. For instance, RF sensors are critical for electron paramagnetic (spin) resonance spectrometers (EPR/ESR) and dielectric spectrometers (DS), including EPR/ESR and DS imaging systems. These sensors usually operate at transmission, reflection, or resonance modes. Existing RF sensors that cover a broad frequency range have low sensitivities. Those that have high sensitivities use resonators and operate at single frequencies or limited frequency points. Previous studies show a transmission coefficient as low as approximately −80 dB and a corresponding effective quality factor as high as approximately 104 with liquid samples. The quality factor for such RF sensors needs to be further improved for applications like measuring single nano-particles, viruses, and molecules. Moreover, broadband operations are needed since many material properties need broadband RF measurements to investigate.
The use of a wide-band 180° splitter has helped to expand the operating frequencies of RF sensors. The frequency extension of such sensors, however, remains relatively modest and the sensor sensitivity is not much improved. Other approaches have achieved higher sensitivity but over a limited frequency range. For instance, dielectric resonators that operate with whispering-galley-modes have reported high quality factors, but only for a single resonant frequency. Moreover, the quality factor can be significantly reduced when lossy material-under-test (MUT), such as biochemical liquids, are introduced. Tunable RF resonators and harmonic-frequency/multi-mode resonator operations can help address the frequency limitation issue. The quality factors, however, remain limited.
Thus, a need exists for a simple RF sensor that can simultaneously provide both increased sensitivity and broadband frequency tuning capabilities.